In most product filling operations, packagers generally fill the product to a level somewhat below the container's highest level. In other words, product volume is generally less than total available container volume. Packagers often refer to differences between product volume and container volume as headspace.
Maintaining container headspace is often desirable for two basic reasons. First, packagers prefer to fill the container based on a pre-measurement of product weight or product volume, for example, a product weight of 500 grams or a product volume of 750 milliliters. Headspace provides a tolerance for subtle differences in product density or container capacity. Second, and perhaps more important, container headspace enables the packager to minimize waste and mess from spillage and overflow of liquids on a high-speed package filling line. High-speed filling lines will generally shake and jostle the filled container risking spillage before the container is sealed. Spillage is a particular concern for wide-mouth containers. Furthermore, spillage can undermine a packager's need to assure consumers that the package contains a full measure of product.
The industry generally considers as wide-mouth any bottle or jar having an opening approximately 50, percent or more in size relative to the container's width or major diameter. In contrast, bottles having an opening substantially less than 50 percent are narrow-neck. As a percentage of overall bottle capacity, wide-mouth bottles tend to have and require more headspace than a narrow-neck version of otherwise similar proportions. Narrow-neck bottle geometry naturally reduces headspace. Moreover, less risk of spillage from the narrow-neck bottle allows packagers to position the fill-level nearer the top further reducing headspace.
Headspace contains gases that in time can damage some products or place extra demands on container structural integrity. Examples include products sensitive to oxygen and products filled and sealed at elevated temperatures.
Filling and sealing a rigid container at elevated temperatures can create significant vacuum forces when excessive headspace gas is also present. Accordingly, less headspace gas is desirable with containers filled at elevated temperatures, sometimes known as “hot-fill,” to reduce vacuum forces acting on the container that could compromise structural integrity, induce container stresses, or significantly distort container shape.
On the other hand, rigid containers experience less internal pressure during pasteurization and retort processes with excessive headspace gas. In-container pasteurization and retort processes involve filling the container first, sealing, and then subjecting the package to elevated temperatures for a sustained period. Metal cans are an example of a package often with excessive headspace.
Interestingly, more flexible polymer containers with minimum headspace gas do not experience significant pressure increases during the pasteurization and retort processes, as is the case with rigid containers. This result is from a greater thermo-expansion of the polymer or plastic relative to rigid glass and metal. This expansion changes the internal volume of the container enough to minimize internal gas pressure increases. Consequently, extra headspace desirable in rigid containers is undesirable in flexible, less rigid containers subjected to pasteurization or retort process.
Traditionally, packagers considered only glass and metal materials for packaging oxygen sensitive products and/or products filled and sealed at elevated temperatures. Both glass and metal materials are relatively low cost, provide an excellent gas barrier, are stiff and generally maintain size and shape, and adequately resist the elevated temperatures found in hot-fill, pasteurization, and retort processes.
On the other hand, metal containers are not transparent and have limited size configuration. Glass containers are heavy often weighing nearly as much as the product. Nonetheless, near perfect gas barrier performance of glass and metal materials minimizes concern for oxygen trapped in the headspace and for some applications minimizes concern from excessive headspace volume.
Until recently, packagers have not seriously considered versatile and ultra lightweight polymer or plastic materials for demanding oxygen sensitive and hot-fill product applications, particularly wide-mouth bottle and jar applications. This is because polymers are generally imperfect barriers to oxygen. Nonetheless, the industry now has a variety of polymers that are well equipped to deal with the practical demands made by oxygen sensitive foods and the marketplace. Those skilled in the art of plastics packaging readily recognize acrylonitrile, nylon or polyamide, ethylene vinyl alcohol, and polyesters, such as, polyethylene naphthalate, modified polyethylene terephthalate, and polyethylene terephthalate copolymers, and many other polymers and polyesters as examples having excellent passive gas barrier performance either individually or as part of a multilayer structure. Some polymers and materials added to polymers create an active gas barrier. Active gas barriers seek out and absorb free oxygen before oxidation of the packaged product occurs.
Manufacturing methods to create multilayer structures of two or more polymers and heat-set techniques to thermally stabilize the container and improve crystalline structure of certain polymers are well known. These techniques play a role enhancing package performance.
Those skilled in the art are aware of several container manufacturing heat-set processes for improving package heat-resistant performance. In the case of the polyester, polyethylene terephthalate, for example, the heat-setting process generally involves relieving stresses created in the container during its manufacture and to improve crystalline structure. Typically, a polyethylene terephthalate container intended for a cold-fill carbonated beverage has higher internal stresses and less crystalline molecular structure than a container intended for a hot-fill, pasteurized, or retort product application. Advanced heat-set approaches include processes disclosed in U.S. Pat. Nos. 6,485,669 and 6,514,451, and U.S. patent application Ser. No. 09/607,817, which are incorporated herein by reference.
Moreover, packagers are more sophisticated and better able to manage product distribution channels. In turn, packagers are now able to define package performance requirements and focus needs case-by-case that enable polymer or plastic based solutions not previously considered practical.
While providing excellent performance, polymers still do not provide a perfect solution. For many product applications, removal of headspace oxygen will often make a difference between package failure and success. The following realistic but hypothetical example illustrates this point.
The amount of oxygen a packaged product can tolerate governs its acceptable shelf life. Air is the headspace gas found most often in sealed containers and contains approximately 21 percent free oxygen. A bottle containing 48 ounces (1362 grams) of product and approximately 30 milliliters of headspace has an oxygen-to-product ratio of about 6.6 parts per million (PPM), assuming no other oxygen sources. Assume the 30 milliliters of headspace is the minimum volume that reasonably minimizes spillage during filling-line handling. The product of this example has an acceptable quality limit of 30 PPM oxygen or less. Higher levels of product oxidation will generally cause noticeable changes in color and/or changes in taste. Assume further that the rate of oxygen ingress into the bottle is about 35 PPM per year. Consequently, the headspace oxygen coupled with oxygen ingress, will grant a product shelf life of about 263 days. However, remove headspace oxygen, and acceptable product shelf life will increase 19 percent to about 313 days.
One solution for modifying headspace atmosphere or removing headspace oxygen is a nitrogen flush. This approach usually involves the addition of one or more drops of liquid nitrogen onto the just filled product immediately before applying the closure and seal. The liquid nitrogen vaporizes expelling the air with its oxygen. While effective, the timing and quantity of liquid nitrogen added is very critical when applied to a lightweight plastic container. Consistency is often difficult to achieve. Too much nitrogen creates internal pressure often giving the plastic container a somewhat bloated appearance. Too little nitrogen is ineffective at expelling the air thus allowing oxygen to remain that shortens product shelf life. Furthermore, the nitrogen flush approach requires additional equipment that many packagers are reluctant to acquire.
Packagers using a polymer container, particularly a wide-mouth container, to hold oxygen sensitive products, need a simple method for allowing the benefit of headspace during product fill, minimizing spillage and displacing headspace, minimizing distortions from vacuum forces and/or product deterioration from oxygen.